Answer:
She buy <u>0.5 pounds</u> of black olives and <u>3.5 pounds</u> of green olives.
Step-by-step explanation:
Given:
Susan is buying black and green olives from the Olive bar for her party. She buys 4 pounds of olives.
Black olives cost three dollars a pound. Green olives cost five dollars a pound.
She spends $15.50.
Now, to find the pounds of olives she buy.
Let the pounds of black olives be 
And the pounds of green olives be 
So, total pounds of olives:

........( 1 )
As, given the cost of black olives $3 a pound.
And cost of green olives $4 a pound.
Now, the total money spends:

Substituting the value of
from equation (1) we get:



<em>Subtracting both sides by 12 we get:</em>

<u>The green olives = 3.5 pounds.</u>
Now, substituting the value of
in equation (1):

<u>The black olives = 0.5 pounds.</u>
Therefore, she buy 0.5 pounds of black olives and 3.5 pounds of green olives.